sometimes not just that, maybe the environment, but that would be a second step after you have someone in custody. the focus of the investigation changes from trying to identify a suspect or a location to actually looking to see if there's material which connects that particular item, in this case a knapsack, to an individual, with their clothing through fibers, or the environment with various dust particles there, whether it's pollen grains that put them in a particular location, the minerals that are present, or even chemicals from bomb-making. all these little particles stick around. what makes trace evidence so valuable is the fact that you can't see it, you're not aware of it, and there's almost no way to protect against it. >> greta: skip, thank you. >> you're welcome. >> greta: well, the pictures, the bags, the fibers and bombs, they are all giant potential clues. right now investigators are rebuilding the bombs from fragments. some are very small. the circuit board could also provide more information. kevin miles join us. kevin, why is the circuit board of any value in an inves